I have two PB slip covered sofas, and have not had this problem. Mine have fairly heavy cotton canvas covers, and when I bought a new cover they warned me off certain fabrics. I wonder if the fabric is too light or "stretchy?" I would use the Velcro advice. They make it with glue on the back.

I don't have it but you can buy large strips of velcro at any fabric/craft store. That may help. Or sew a few stiches in to keep it in place and just resew when you wash it. Or safety pins.

We have a PB Basic Slip-covered Sleeper Sofa. I'm on my second slip cover and I had the same issue with both. The front part of the cover, the skirt under the seat cushions--it sags. I periodically take off the seat cushions and yank it up so that the cord/seam is straight again, but then it starts sagging again. Has anyone found a solution to this problem?